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To Deliver Compressed Samples with the Asynchronous Reader

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[The feature associated with this page, Windows Media Format 11 SDK, is a legacy feature. It has been superseded by Source Reader and Sink Writer. Source Reader and Sink Writer have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use Source Reader and Sink Writer instead of Windows Media Format 11 SDK, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

The asynchronous reader can deliver compressed samples from streams in ASF files. Applications usually deliver compressed samples when copying a stream from one file to another. It is not advisable to recompress data that has been reconstructed from a compressed stream, because data is lost in the encoding process. Digital media that has been compressed more than once will have a noticeable decrease in quality.

The Windows Media Format SDK does not provide any methods for decoding data after it has been extracted from an ASF file. If you receive compressed samples and later want to decompress them, you will have to provide your own code to do so. One way to get around this limitation is to write the compressed samples to a new ASF file and then re-read them into normal, uncompressed samples.

To receive compressed samples with the asynchronous reader, perform the following steps.

  1. Implement the IWMReaderCallbackAdvanced::OnStreamSample callback. This callback is basically identical in function to IWMReaderCallback::OnSample except that it delivers samples by stream number and the samples are still compressed.
  2. Before starting playback, obtain a pointer to the IWMReaderAdvanced interface of the reader object by calling IWMReader::QueryInterface.
  3. Configure the reader to deliver compressed samples for the desired stream by calling IWMReaderAdvanced::SetReceiveStreamSamples.
  4. Repeat step 3 for each stream for which compressed sample delivery is desired.

Note

Image streams are not valid for compressed stream delivery. If you copy an image stream from one file to another, it will not work in the new file. To copy an image stream from file to file, retrieve the image stream samples by output number and include them in the new file as if including a new image stream.

 

IWMReaderCallbackAdvanced Interface

Reading Files with the Asynchronous Reader